This invention relates to a method of making 2,4-dichlorotoluene (24DCT) and 3,4-dichlorotoluene (34DCT). In particular, it relates to a method in which parachlorotoluene (PCT) that contains some metachlorotoluene (MCT) is brominated then chlorinated without separating the brominated products from the unbrominated products.
Commercial PCT is made by chlorinating toluene. After distilling off the unreacted toluene and the orthochlorotoluene (OCT), the product is primarily PCT, but small amounts of MCT and OCT are also present, typically about 0.5 to about 1 wt % MCT and about 0.5 to about 1 wt % of the OCT. While OCT can be removed by distillation and the presence of small amounts of the OCT is usually innocuous anyway, it has been found that the presence of MCT can deleteriously affect the properties of the chemicals made from PCT, such as pharmaceuticals, paint pigments, and herbicides. Unfortunately, the boiling point of MCT is close to the boiling point of PCT and the two isomers cannot be easily separated. U.S. application Ser. No. 09/193,755, of which this application is a continuation-in-part, discloses a method of separating PCT from MCT.
If PCT is further chlorinated and no MCT or OCT is present, 24DCT and 34DCT will be the only DCT formed. (If OCT and PCT are present, 4 additional DCTS having similar boiling points can be formed.) Since 24DCT boils at 201.degree. C. and 34DCT toluene boils at 209.degree. C., they can be separated by distillation, giving high purity 24DCT and 34DCT. At present, chlorinating high purity PCT, that is, PCT that contains little or no MCT or OCT, is the only way to obtain 24DCT and 34DCT in high purity.